Conventionally, thin-film transistor optical sensors have been used for an electro optical apparatus such as a photocopier or a facsimile. Recently, X-ray image sensors have been developed as display devices related to a hospital information automation system such as PACS (Picture Archiving Communication System).
FIG. 1 is a cross sectional view illustrating an image sensor comprising a conventional thin-film transistor optical sensor. Referring to FIG. 1, a gate electrode 22a of a switching thin-film transistor, a lower common electrode 29a, and a gate electrode 22b of an optical sensor are formed to be separated from each other on an insulating substrate 21. A gate insulating film 23 is deposited above the gate electrode 22a, the lower common electrode 29a and the gate electrode 22b. 
A protective insulating film 27 is disposed above the gate insulating film 23. An intrinsic amorphous silicon layer 24, an N type amorphous silicon layer 25, a drain electrode 26a and a source electrode 26b of the thin-film transistor, a connection portion 26c, and a pixel electrode 26d and a power source electrode 26e of the optical sensor are disposed between the gate insulating film 23 and the protective insulating film 27. In addition, a metal light shielding film 28a is deposited above the right portion of the protective insulating film 27.
In addition, the gate electrode 22b of the optical sensor are commonly connected to gate electrodes of the image sensors in the adjacent array, and a storage capacitor is formed between the pixel electrode 26d and the lower common electrode 29a. 
At the time of the operation of the optical sensor comprising the conventional amorphous thin-film transistor having the above structure, a negative voltage is applied to the gate electrode 22b of the optical sensor to minimize the dark leakage current of the optical sensor. However, there is a problem that, in a high voltage of 10V or more, the dark leakage current is too high to implement a high voltage driving. In addition, since the gate electrode 22b of the optical sensor is overlapped by the upper power source electrode, there is another problem that, in a high power source voltage of 20V or more, the dark leakage current is increasing, so that the dynamic range, a region on which lightness and darkness are able to be distinguished, is reduced.
In addition, as shown in FIG. 2, when the negative voltage is applied to the gate electrode 22b of the conventional amorphous silicon thin-film transistor optical sensor, holes are accumulated in an intrinsic semiconductor layer 24 to form a portion 31 which exhibits properties of a P type amorphous silicon. The gate electrode 22b is overlapped by the pixel electrode 26d and the power source electrode 26e in the optical sensor to form an N-P-N contact together with an N type amorphous silicon 25 so that the dark leakage current can be reduced. However, when a higher voltage is applied to the pixel electrode 26d, a strong electric field is formed between the N type layer and the P type layer, and then a depletion layer is narrowed. Like this, if the depletion layer is narrowed, there is still another problem that a large amount of the leakage current is generated. Because of the above problems, the optical sensor having the conventional structure is not suitable for its high voltage usage.
FIG. 3 is a graph illustrating a relationship between a drain current and a dark leakage current at the time that light is incident on a gate electrode of the conventional thin-film transistor optical sensor. In case of the conventional thin-film transistor optical sensor, when the gate electrode of the optical sensor ranges from −15V to −5V and the power source voltage of the optical sensor is a low voltage of 10V or less, the dark leakage current of the optical sensor is increased up to about 10−8 A.
Like this, the image sensor comprising the conventional silicon thin-film transistor optical sensor has the problem that the dark leakage current is increasing when a high voltage is applied.